The present invention relates to an injection moulding process, more particularly to a process for manufacturing injection moulded objects of thermoplastic material, to a device for carrying out such processes, and to a new type of moulded objects obtainable with such process.
Injection moulding of thermoplastic materials to produce objects of all sorts of shapes and sizes, is well known in the art.
It is also known however that there are certain limitations to the size of the objects to be moulded (in function also of the complexity of the moulded shapes) as a consequence of the rheologic properties of the molten thermoplastic in the moulds.
As the flow of molten plastic progresses into the mould cavity the material will cool down, which together with the Theological resistances in the mould will cause the pressure to build up at the point of injection; these pressures will of course be related to the length of the flowlines in the mould and therefore to the size of the object to be moulded.
To overcome these problems several solutions have been proposed in the art, such as:
high pressure equipment to overcome the pressure build-up, but this approach is quite expensive due to the material costs involved;
use of heated moulds to avoid the cooling down of the molten plastic in the mould, but this approach is also complex and costly due to the structural complexity of the different parts of such a heated mould; and
use of a plurality of injection points into the mould cavity to increase the speed of injection and the volume of the molten material flowing into the mould so as to reach the other side of the mould and fill the mould before the pressure builds up too much, but this approach has only shown satisfactory where the size of the object to be moulded in the directions) away from the injection points (the xe2x80x9csecond dimensionxe2x80x9d or base area, and the xe2x80x9cthird dimensionxe2x80x9d or height of the three dimensional object) is still relatively limited, i.e., at most about 50-60 cm, and at the absolute most about 80 cm when also using high pressure technology and/or heated mould technology and/or very special thermoplastics.
The object of the present invention is to provide a method for manufacturing injection moulded objects of thermoplastic material which overcomes the drawbacks of the known art and which in particular allows the injection moulding of objects having a size of more than 80 cm in a direction away from the injection point(s), or objects having a size of more than 60 cm in a direction away from the injection point(s) without using high pressure technology. It is also the object of this invention to provide a possibility for objects having a size of more than 80 cm along each of its three dimensional axes. Furthermore, the present invention provides for even flowing of the molten material from all three axes, rather than singular or two-dimensional flow methods that are currently in use. The present invention allows for uniformity along the flow lines and formation of elongate large sized three dimensional structures that is not problematic with existing methods.
The invention therefor provides a process for manufacturing injection moulded objects of thermoplastic material by injecting molten thermoplastic material into a mould from at least two distinct injection points whereas the flows of molten material from each injection point fill part of the mould and flow together to fill the entire mould while still in molten state, wherein at least two distinct injection points are provided at opposite sides of the mould.
The expressions xe2x80x9copposite sidesxe2x80x9d of the mould or xe2x80x9copposite sidesxe2x80x9d of the moulded object, as used herein, should be understood as designating positions on the mould or on the moulded object from which two flows of molten plastic flowing in essentially opposite directions are able to fill the volume of said mould or object.
Whereas according to the state of the art of injection moulding of thermoplastics multiple injection points into one single mould are known, these multiple injection points have been located on essentially one side of the mould whereas the distance between these injection points has been kept relatively close to achieve as quickly as possible the merger of the individual progressing flows of molten material and form one relatively uniform flow of molten material progressing in essentially one direction.
It has now been found surprisingly that by using more than one flow of molten material flowing in essentially opposite directions, in accordance with this invention, whereby the merging of the progressing flows is postponed until the actual filling of the mould at the points of merger of these flows, moulded objects of excellent quality could be obtained, contrary to the generally admitted prejudice that the individual flows should merge together as soon as possible to avoid quality loss at the merging line of the individual flows of molten material.
The injection of the molten material at the separate injection points located at opposite sides of the mould is preferably carried out substantially simultaneously, but under specific circumstances depending on the shape and structure of the mould and the internal flowlines in the mould it may be preferable to start with the injection at one of the opposite injection points and only subsequently start injection at the other injection point(s), in a pre-established timing sequence that may readily be determined experimentally by the skilled art worker in function of the particularities of the object to be moulded.
Thus also, the actual position of the opposite injection points to the mould are essentially determined by the length of the overall flowlines of the molten material in the mould structure whereby the injection points are preferably positioned in such way that the length of the flowlines or the flowtime of the molten material from the opposite injection points on the mould to the point of merger of the molten material from the opposite injection points is essentially evenly distributed.
Molten material is injected into a mould from plural positions on the mould, allowing for plural flows of molten material through the mould. The flows flow in opposite directions from three dimensional axes of the mould from injection points in one plane along two dimensional axes of the mould, and from at least one further injection point along a third dimensional axis perpendicular to the one plane, thus creating three dimensional injection of molten material into the mould. This allows for even flowing of the molten material from all axes.
This also will be easily established by the skilled art worker in function of the particularities of the objects to be moulded.
The number of injection points in accordance with the invention is at least two, but this number may be more as required by the particularities of the objects to be moulded.
Thus there may be used three or more injection points from different angles and extremities of the mould to provide more than two flows of molten plastic material flowing in essentially opposite directions.
In this context it should be observed that for instance four injection points essentially located according to the four summits of a tetraeder are considered to provide flows of molten material globally flowing in essentially opposite directions before merging together at a more or less central point of the mould.
The injection points at opposite sides of the mould may furthermore also each consist of a plurality of injection points on each respective opposite side of the mould.
According to another embodiment of the invention xe2x80x9clow pressurexe2x80x9d technology is preferably used to produce objects having a size of more than 60 cm along each of its three dimensional axes.
The expression xe2x80x9clow pressurexe2x80x9d technology as used herein refers to the use of injection pumps allowing a maximum cavity pressure of 100 bar, as opposed to xe2x80x9chigh pressurexe2x80x9d, technology which uses injection pumps with maximum cavity pressures well above 250 bar.
It has also been found surprisingly that with the present invention it is easy to avoid any problems of air entrapment between flows of molten material merging from opposite directions, contrary to what would have been expected in view of the state of the art prejudices.
According to a specific feature of the invention this is achieved by allowing the air displaced in the mould by the flows of molten thermoplastic material to escape through slits which allow through the air but not the molten thermoplastic material.
In practice it has been found that the slits and interstices anyway existing in standard good quality multi-piece moulds provide the necessary properties for this preferred feature of the invention.
According to still another feature of the invention, the thermoplastics used are preferably polyolefins such as in particular polyethylenes (low density polyethylenes, high density polyethylenes, linear low density polyethylenes, etc.) and polypropylene; the process is however also suitable for any other type of thermoplastic resins.
The invention also relates to devices for carrying out the injection moulding process according to the invention.
In particular the invention thus relates to an injection moulding equipment for the manufacture of moulded objects of thermoplastic material, comprising at least two distinct injection points provided at opposite sides of a single mould assembly.
According to a preferred feature of the invention, the equipment is specifically designed for the injection moulding of polyolefin objects having a size of more than 80 cm along each of its three dimensional axes.
In a preferred embodiment of the injection moulding equipment according to the invention the equipment is of the low pressure technology type. In this embodiment the equipment is preferably designed for the injection moulding of polyolefin objects having a size of more than 60 cm along each of its three dimensional axes.
According to another preferred feature of the invention, the mould assembly of the injection moulding device comprises slits of a size allowing through air but not the molten thermoplastic material.
As already stated above standard good quality multi-piece moulds generally provide the necessary properties for this preferred feature of the invention.
The invention finally also relates to three dimensional moulded objects of thermoplastic material with features unachievable and therefore unknown before in the art, i.e. having a size of more than 80 cm along each of its three dimensional axes.
More in particular the invention relates to moulded thermoplastic cases with a bottom and a peripheral wall or peripheral walls, with length and width axes or diameter of the bottom and a height of the walls of at least 80 cm, preferably of at least 100 cm.
According to a preferred feature of the invention these three dimensional moulded objects are made of a thermoplastic polyolefin, preferably polyethylene, polypropylene or mixtures thereof.
The invention will be further explained herebelow with reference to the attached drawings representing specific embodiments of the invention. It should be observed that the specific features of these embodiments are only described as preferred examples of what is intended within the above general disclosure of the invention and should by no means be interpreted as limiting the scope of the invention as such and as set forth in the claims.